Heavy duty machines such as mining trucks consume tremendous energy. Rising fuel costs and pressing environmental concerns call for improvement in fuel efficiency for these machines. For example, regenerative braking is an effective means to improve fuel efficiency for electrical drive mining trucks. During regenerative braking, a motor of the truck may function as a generator to slow the movement of the truck by converting the kinetic energy of the truck into electrical energy. The electrical energy is then supplied to a battery, a power supply, another motor, etc.
Part of the regenerated electrical energy may be absorbed by capacitors and resistors connected to a DC bus. This causes the DC bus voltage to rise. Thus, as a mobile vehicle repeatedly undergoes regenerative braking, the DC bus voltage may also fluctuate frequently in a wide range. Because a motor draws energy from the DC bus, such fluctuations of the DC bus voltage may significantly impact the amount of torque that can be generated by the motor. Unless the motor can efficiently use each DC bus voltage level to generate a desired torque, the overall fuel efficiency of the mobile vehicle will be undermined.
Vector control strategy can control torque in response to the DC bus voltage. U.S. Pat. No. 4,388,577 (the '577 patent) issued to Blaschke et al. on Jun. 14, 1983, discloses a system using a vector control strategy to control torque output in response to DC voltage fluctuation. The system of the '577 patent decomposes the stator field vector into a component perpendicular to the rotor field vector, and a component parallel to the rotor field vector. The system also uses space vector modulation to align the rotor field vector in phase with a stator field vector. At a given DC bus voltage, the system uses a feedback circuit to change the perpendicular component to a model perpendicular component. The model perpendicular component is a pre-calibrated value as a function of the DC bus voltage.
Although the system of the '577 patent may provide a solution to control motor torque output in response to a changing DC bus voltage, its reliance on the model perpendicular component may be unnecessarily complicated and may not be an efficient use of a specific DC bus voltage. In particular, the model perpendicular component needs constant calibration, which is time consuming. Moreover, the parallel component still exists and thus part of energy drawn from the DC bus is wasted in supplying this parallel component.
The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.